Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. Various engine systems may be used with CNG fuels, utilizing various engine technologies and injection technologies that are adapted to the specific physical and chemical properties of CNG fuels. For example, mono-fuel engine systems may be configured to operate with CNG while multi-fuel systems may be configured to operate with CNG and one or more other fuels, such as gasoline or gasoline blend liquid fuels. Engine control systems may operate such multi-fuels systems in various operating modes based on engine operating conditions.
However, CNG engines, particularly engines that have been converted to run on CNG, may experience numerous operating problems. CNG has marginal ignitability and a narrow rich limit compared to gasoline and other traditional fuels. Thus, when running a CNG engine at high loads, the temperature limit of the engine may be reached prior to fully combusting the fuel or air taken into the combustion cylinder. By not fully combusting the contents of the combustion cylinder, the likelihood of engine knock increases. Further, the combustion of CNG produces less soot than for an equivalent gasoline engine. This reduces the natural lubrication of engine valves, potentially leading to valve recession and degradation.
At peak operation, rich running gasoline engines may operate up to 40% rich to abate overheating. In comparison, NG engines operate around 10% rich at peak conditions. The inventors therefore developed a method to lower the air-fuel ratio (AFR) in natural gas engines so as to provide some of the valve protection that is present in gasoline engines.
To counter overheating problems, the air charge may be limited by either throttling airflow or running lean, but these solutions may limit the maximum power output of the engine. Specific power may be increased by increasing the size of the engine, but this may not be possible for all platforms or conversions. Injecting water or other control fluids into the combustion chamber may reduce temperatures and guard against engine knock, but may further reduce the ignitability of the fuel mixture.
CNG engines also experience increased valve wear for a number of reasons. Natural gas has a higher specific heat than gasoline and thus burns at a higher temperature. Natural gas also has a significantly smaller hydrocarbon concentration than gasoline engines. The inventors therefore developed a method to increase the operational AFR richness in natural gas engines so as to provide some of the valve protection that is present in gasoline engines.
The inventors herein have realized that the above issues may be at least partly addressed by injecting an amount of secondary fuel with a higher AFR rich operating limit into the combustion chamber or gaseous fuel source, the AFR at which the engine can operate may increase. Added liquid fuel also introduces higher latent cooling because heat energy is absorbed in the evaporative process. Liquid fuel may also acts as a diluent to lower flame temperature during combustion. A richer AFR also allows for more advanced spark timing not available in lean burning natural gas engines that, in combination with lower heat generation, also help to reduce engine knock tendency. Further, using a secondary fuel source with a higher hydrocarbon concentration allows for increased soot production that acts both as a valve lubricant, microwelding, and a thermal barrier thus abating valve recession. Using steam reformation, CNG may be reformed to provide secondary fuels such as CO and H2 in some embodiments.
The inventors further realized that the above issues may be at least partly addressed for example, by a method for a turbocharged engine, comprising: during high load conditions, in response to an elevated engine temperature, after port injecting a first quantity of a first gaseous fuel, direct injecting a second quantity of a second, liquid or gaseous fuel at a first timing that is a function of a desired air-fuel ratio (AFR). In this way, engine power for an engine primarily fueled by the first, gaseous fuel may be maximized while simultaneously controlling the maximum combustion temperature and maximum combustion pressure and mitigating engine knock.
In another example, a method for a turbocharged engine may comprise: during high load conditions, in response to an elevated engine temperature, after port injecting a first gaseous fuel, direct injecting a second, liquid fuel at a timing that is after combustion spark-ignition, but during combustion of, the first gaseous fuel. In this way, a second, liquid fuel injected between spark ignition and a top-dead center event may reduce combustion temperature and pressure, regardless of the ignitability of the second, liquid fuel. Further, a second, liquid fuel injected after spark ignition and following a top-dead center event may reduce exhaust temperatures regardless of the ignitability of the second, liquid fuel. Liquid fuel also introduces higher latent cooling as heat energy is absorbed in the evaporative process of liquid fuel. The inventors also found that high hydrocarbon concentration of some liquid fuels (in comparison to natural gas) increases soot production that acts as a valve lubricant, microwelding, and thermal barrier to reduce valve wear.
In yet another example, a method for a turbocharged engine, comprising: during high load conditions, in response to engine knock, after port injecting a first gaseous fuel, direct injecting a second fuel while maintaining spark timing. In this way, engine knock in an engine primarily fueled by a gaseous fuel may be mitigated by injecting a second, liquid fuel coincident with combustion events, and without iteratively advancing and retarding spark timing in response to engine knock.
Further disclosed herein are systems for providing secondary fuel to the engine. For example, an embodiment may use a reformation catalyst within an EGR system to provide an intake with a gaseous secondary fuel source with a higher AFR richness operating limit. Other embodiments may use additional liquid fuel or gaseous tanks to provide a secondary fuel to the intake system. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.